1. Field of the Invention
The present invention relates to a crankshaft and to a method for producing the same, and particularly relates to a technique for forming a hollow hole in a crankpin of the crankshaft.
2. Related Art
An internal-combustion engine is provided with, for example, a crankshaft 10 as shown in FIG. 10. The crankshaft 10 is provided with a journal shaft 11, and a crankpin 13 that is parallel to the journal shaft 11 is connected thereto via an arm 12. A counterweight 12A is formed at the arm 12, and a forming position of the counterweight 12A with respect to the journal shaft 11 is at the opposite side to a connecting position of the crankpin 13.
In the journal shaft 11, a journal shaft side through hole 21 for supplying an oil to a surface of the journal shaft 11 is formed. In the crankpin 13, a pin side through hole 22 for supplying oil to a surface of the crankpin 13 is formed. The journal shaft side through hole 21 and the pin side through hole 22 are connected via an oil flow passage 23. The oil in the journal shaft side through hole 21 is supplied to the surface of the crankpin 13 from the pin side through hole 22 passing through the oil flow passage 23. Reference numeral 24 is a plug 24 for closing an opening of the oil flow passage 23.
In the crankshaft 10 having the above structure, various techniques are proposed for weight saving from the viewpoint of improvement of fuel efficiency. For example, it is proposed that the surface of the arm 12, which is near the crankpin 13 and a journal shaft 11, is entirely formed with recessed grooves (for example, Japanese Patent Application, First Publication No. 2005414131). Furthermore, it is proposed that a hollow hole is formed in the crankpin 13. In this case, the hollow hole is locally formed, so that rigidity of the crankshaft 10 is not so degraded. Therefore, the hollow hole is preferably formed in the crankpin 13.
For forming the hollow hole, it is proposed to use a forging apparatus provided with a side forming punch that moves perpendicularly with respect to a moving direction of a press ram (for example, Japanese Utility Model Application, Publication No. 61-143727). In this technique, plural holes cannot be simultaneously formed, so that die sets are prepared according to the forming positions of the holes and the holes are individually formed, and therefore, this technique is very complex.
This is a reason why the hollow hole is formed after trimming in a condition in which the crankshaft 10 shown in FIG. 11 is not disposed in a closed space. Specifically, for example, when a hole is formed in the crankshaft 10, as shown in FIGS. 12A to 12C, one surface of one arm 12 (the rightmost arm 12 in FIG. 11) is abutted at a lower die 1 (the shaded area in the Figures) and a punch 2 is inserted into the crankpin 13 from an upper surface side of another arm 12 connected by the crankpin 13. In this case, a mark of abutting may remain on the abutting portion of one surface of one arm 12 (the rightmost arm 12 in FIG. 11) with respect to the lower die 1.
In an internal-combustion engine such as an automobile engine, a crankshaft by which a reciprocating movement of a piston is converted into a rotational movement via a connecting rod is used. The crankshaft is provided with a journal shaft and a crankpin that is parallel to the journal shaft is connected thereto via an arm. A counterweight is formed on the arm and a forming position of the counterweight with respect to the journal shaft is at the opposite side to the connecting position of the crankpin.
The crankshaft is mainly produced by forging using an upper die and a lower die that are separatable. In the forging, a heat-treated material is disposed in a forging press and the material is subjected to various forming using various upper dies and lower dies in the forging press, so that the crankshaft can be obtained.
In automobiles, weight saving of the engine is required for improvement of fuel efficiency, so that a weight of a crankshaft used in the engine is reduced. In weight reduction of the crankshaft, a technique in which a material of the crankshaft is strengthen by modifying quality of the material and a hardening treatment, thereby obtaining a reduced cross section, is mainly used. In this technique, as a material of the crankshaft in the forging, a carbon steel is mainly used and a special steel is used in parts. Furthermore, these materials are partially subjected to high-frequency hardening and a heat-treatment such as a nitriding treatment, if necessary.
A technique for hollowing inner portions of a crankpin and a journal is proposed instead of the above technique in which a hollow crankshaft is formed in a shape having a reduced cross section. In the technique for hollowing the crankshaft, portions except for an oil flow passage are hollowed in an axial direction, so that the hollow crankshaft is much lighter than a solid crankshaft.
As a hollowing technique, a technique in which a hollow hole is formed in a crankpin by boring using a drill is known. A shape of the hole is circular or oval in this technique. When the shape of the hole is complicated, various drills are used in the boring and machining time is extended, so that selectivity in the shape of the hole is decreased.
Therefore, hollowing a crankshaft by forging may be applied. In conventional forging apparatuses, parts of an upper die and a lower die are designed in separatable structures and removal of excess materials created in forming a hole is difficult, so that side forming may be applied. For example, in a forging apparatus disclosed in Japanese Utility Model Application, Publication No. 61-143727, a side forming punch is perpendicularly moved with respect to a moving direction of a press ram by using a cam mechanism moving in conjunction with the press ram, so that a hollow hole is formed in a crankpin.
A hole 13A and a hole 13B are individually formed by the above-explained insertion of a punch 2. In this case, as shown in FIG. 12A, when the punch 2 is inserted from a direction parallel to the journal shaft 11, an upper portion of the journal shaft 11 is deformed toward the lower direction according to the insertion of the punch 2 in forming the hole 13A. In forming the hole 13B, interference with an adjacent portion to the arm 12 occurs, so that the punch 2 cannot be inserted.
As shown in FIG. 12B, when outer circumferential surfaces of the journal shaft 11 and the counterweight 12A of the arm 12 are abutted at the lower die 1 and the punch 2 is inserted from the perpendicular direction with respect to the journal shaft 11, the arms 12 connected by single crankpin 13 to each other are deformed toward the lower direction. As shown in FIG. 12C, in inclining the lower die 1 at a predetermined angle, when the outer circumferences of the journal shaft 11 and the counterweight 12A of the arm 12 are abutted at the lower die 1 and the punch 2 is inserted from the predetermined angular direction, not only are the arms 12 connected by single crankpin 13 to each other deformed toward the lower direction, but also inclining deformation toward the inclining direction of the lower die 1 occurs in the counterweight 12A.
In these techniques shown in FIGS. 12A to 12C, portions of the crankshaft 10 and crankpin 13 except for portions abutting at the lower die 1 are not restrained, so that these portions not abutting thereto are deformed. In this case, surface shrinkage in the axial direction of the crankpin 13 occurs on end surfaces around openings of the holes 13A and the 13B of the crankpin 13 and burrs are formed at the edges around the openings thereof. Therefore, predetermined dimensional accuracy of the crankshaft 10 cannot be obtained, and an operation for removing the burrs is added. Furthermore, major balancing correction such that a large number of holes (not shown in Figures) are formed in a counterweight 12A of the arm 12 is required, so that production cost is increased. When the holes 13A and 13B are deformed, a defective portion is formed in machining the oil flow passage 23 (shown only in FIG. 10) after forming the holes 13A and 13B.